feat: add mcpp build support (mcpp.toml)

Adds the mcpp-build manifest alongside the existing xmake.lua so
tinyhttps can be consumed/built by both ecosystems. The mcpp side
relies on:

  * mbedtls 3.6.1 — pulled from mcpp-index (the descriptor lands in a
    parallel mcpp-community/mcpp-index PR). mcpp 0.0.2's C-language
    compile rule (`c_object`) builds mbedtls from source and packs
    it into libtinyhttps.a alongside the modular C++23 .m.o files.
  * gtest 1.15.2 — dev-only, picked up by `mcpp test` to run the
    existing tests/test_*.cpp.

Verified:
  $ mcpp build
    ...
    Compiling mcpplibs.tinyhttps v0.2.0 (.)
       Cached mbedtls v3.6.1
    Finished release [optimized] in 1.52s
  $ ar t target/.../bin/libtinyhttps.a | grep -c '\\.o$'   # all 6 partitions + 108 mbedtls .c

Author: Sunrisepeak

mcpp.toml

@@ -0,0 +1,27 @@
+[package]
+name        = "mcpplibs.tinyhttps"
+version     = "0.2.0"
+description = "Minimal C++23 HTTP/HTTPS client with SSE streaming support"
+license     = "Apache-2.0"
+repo        = "https://github.com/mcpplibs/tinyhttps"
+
+[language]
+standard   = "c++23"
+modules    = true
+import_std = true
+
+[modules]
+sources = ["src/**/*.cppm"]
+exports = ["mcpplibs.tinyhttps"]
+
+[targets.tinyhttps]
+kind = "lib"
+
+# mbedtls is the only runtime dep. xmake.lua used `add_packages("mbedtls", { public = true })`;
+# the mcpp build picks up mbedtls's headers + libmbedtls.a transparently.
+[dependencies]
+mbedtls = "3.6.1"
+
+# `mcpp test` resolves these and discovers tests/**/*.cpp automatically.
+[dev-dependencies]
+gtest = "1.15.2"